Amine-functional copolymers are well known, and these have been rendered cationic and water dispersible by neutralization with a solubilizing acid. The dispersed resin particles migrate to the cathode when a unidirectional electrical current is passed through the aqueous medium which contains the resin particles, and these particles deposit upon the cathode to form a coating thereon. The coated cathode is removed from the electrocoating bath and then baked to cure the coating.
In the usual practice, the amine-functional copolymer contains reactive groups, such as hydroxy groups, and an aminoplast resin curing agent is incorporated in the bath for codeposit with the reactive amine copolymer, and the aminoplast resin cross-links the copolymer when the coating is baked. Melamine-formaldehyde condensates have been used, but the corrosion resistance and detergent resistance have not been fully satisfactory. Benzoguanamine-formaldehyde condensates provide better properties in the cured films, but these are much more expensive. In either event, precise proportional deposit of the amine resin and the aminoplast resin is hard to achieve, and this complicates the electrodeposition process.
It has also been disclosed to incorporate N-methylol-functional reactive groups in the amine copolymer so as to provide a self-curing cationic copolymer which would enable the use of a curing coreactant, such as the aminoplast resins normally used, to be eliminated. However, and despite the obvious simplification and economy provided through the use of self-curing resins, these self-curing cationic systems have not been successful in industry, probably because it has been difficult to combine the curing reactivity and bath stability which is needed to enable a system to be successful.
The prior efforts using self-curing copolymers are summarized in U.S. Pat. Nos. 3,446,723, 3,454,482, 3,455,806 and 3,458,420. The thrust of these patents is to use not more than 30%, based on the weight of the copolymer, of the N-methylol-functional monomer in the copolymer.
It is stressed that large amounts of N-methylol functionality is needed in order to cure the low molecular weight copolymers to the point where they possess good properties. On the other hand, the more N-methylol functionality, the more undesirable prereaction during copolymerization and storage in the aqueous acidic medium of the electrocoating bath. On the other hand, if etherification is used to stabilize the N-methylol functional groups and prevent their prereaction, then the these ether groups either hydrolyze in the aqueous medium to cause prereaction, or the hydrolysis-resistant ether groups prevent or reduce water dispersibility.